As shown in FIGS. 8 and 9, a chain tension-imparting device 500, recently developed by Tsubakimoto Chain Co., of Osaka, Japan, comprises a generally cylindrical plunger 520, which protrudes toward a traveling chain (not shown); a housing body 510, having a plunger-receiving hole 511, in which the plunger 520 slides retractably; a protrusion biasing spring 530, which biases the plunger 520 in the protruding direction with respect to the housing body 510; a cam-receiving ring 540, fitted around plunger 520 and received in an enlarged end portion 511a of the plunger-receiving hole; a spring 550 for biasing the cam-receiving ring 540 in the protruding direction; a pair of wedge-shaped cam chips 560, which slide on sloping surfaces 541 of cam guiding grooves formed in the cam-receiving ring 540 and engage a pair of racks 521 formed on opposite sides of the plunger 520; a cam guide ring 570, fitted on the outside of the plunger 520 in the enlarged end portion 511a of the plunger-receiving hole 511, for guiding and controlling disengagement of the pair of wedge-shaped cam chips 560 from the racks 521; and a seal plate 580, which slidably fits the plunger 520 and seals the biasing spring 550, the cam-receiving ring 540, the cam chips 560, and the cam guide ring 570, all of which are disposed in the enlarged end portion 511a of the plunger-receiving hole 511. The tension-imparting device is configured so that when the chain becomes elongated, the plunger 520 is advanced sequentially, by one tooth at a time, to restrict the backlash distance of the plunger, so that proper chain tension is maintained, and noise, which would otherwise occur in the chain, is prevented. This device is described on page 1, and shown in FIGS. 2 and 3, of Japanese Patent Application No. 2002-338124.
Since the plunger 520 is biased to protrude from the housing body 510 toward a traveling chain, it is difficult to hold the plunger in its retracted condition during mounting of the tension-imparting device. The objects of the invention are to solve the above-mentioned problem, and to provide a simple chain tensioner which reliably, and automatically, shifts from a condition in which the protrusion of the plunger is blocked to facilitate mounting, to a condition in which the plunger is released for normal tension-imparting operation.